In electrophotography an image comprising an electrostatic field pattern usually of non-uniform strength (also referred to as an electrostatic latent image), is formed on an insulative surface of an electrophotographic recording element comprising at least a photoconductive layer and an electrically conductive substrate. Several types of electrophotographic recording elements are known for use in electrophotography. In many conventional elements, the active photoconductive or charge-generation materials are contained in a single layer. This layer is coated on a suitable electrically conductive support or on a non-conductive support that is overcoated with an electrically conductive layer. In addition to single-active-layer electrophotographic recording elements, various multiactive electrophotographic recording elements are known. Such elements are sometimes called multi-layer or multiactive-layer elements because they contain at least two active layers that interact to form an electrostatic latent image.
A class of photoconductive materials that has been employed in the aforementioned single-active layer and multiactive elements is titanyl phthalocyanine-type pigments such as titanyl phthalocyanine pigment or titanyl fluorophthalocyanine pigment. Electrophotographic recording elements containing such pigments as photoconductive or charge-generation materials are useful in electrophotographic laser beam printers because they are capable of providing photosensitivity in at least a portion of the near infrared region of the electromagnetic spectrum, i.e. in the range of 700-900 nm.
Unfortunately, electrophotographic recording elements of the prior art which contain photoconductive titanyl phthalocyanine-type pigments have typically suffered from one or more disadvantages that have significantly restricted their use. Thus, without special processing techniques or treatments, neither titanyl phthalocyanine pigment nor titanyl fluorophthalocyanine pigment provides sufficient electrophotographic speed in the near infrared range that is needed in modern-day mid- to high volume laser beam printers and particularly the high electrophotographic speed that is needed at longer wavelengths such as 830-900 nm within such range. For example, vacuum sublimation (also known as vacuum deposition) is often used to deposit titanyl phthalocyanine-type pigments is a form suitable for high speed electrophotographic elements. Vacuum sublimation, however, is a batch process which makes production scale runs quite costly and thin sublimed films are fragile and susceptible to damage until they can be protected by a more durable overcoat.
U.S. Pat. No. 4,701,396, issued Oct. 20, 1987, also points out that photoconductive titanyl phthalocyanine-type pigments are not readily dispersible in liquid coating compositions comprising solvent solutions of polymeric binders which are used to dispersion coat charge generation layers in electrophotographic recording elements. It is necessary that the titanyl phthalocyanine-type pigment be in a form (crystalline or amorphous) that is highly photoconductive and sufficiently and stably dispersed in a coating composition to permit its being applied at a low enough concentration to form a very thin layer having acceptable electrophotographic speed in the near infrared range.
In U.S. Pat. No. 4,701,396, titanyl fluorophthalocyanine pigment is subjected to a treatment which modifies its crystalline form and reduces its particle size so that the pigment can be dispersed in liquid coating compositions comprising a solvent solution of polymeric binder. This treatment is called "acid-pasting" which involves dissolving the pigment (after extraction purification of the as-synthesized material) in cold, concentrated mineral acid, preferably sulfuric acid, and pouring the solution into ice water to re-precipitate the pigment. The precipitate is washed free of acid with water, then with an alcohol and dried. The resulting titanyl fluorophthalocyanine pigment has a substantially smaller particle size (slightly less than 1 micrometer) than the crude pigment and is highly sensitive to radiation in the near infrared range. In commercial scale operations it is, of course, desirable to avoid using large amounts of concentrated mineral acids such as sulfuric acid because of safety and environmental considerations. It is also very costly to provide the necessary safeguards for handling such a hazardous material.
This invention is directed toward the objective of providing electrophotographic recording elements that comprise titanyl phthalocyanine-type pigments and have excellent photosensitivity in the near infrared range without using special coating techniques such as vacuum sublimation or chemical treatments such as acid-pasting.